examination of the spectral quality of downwelling light, as might be expected, it
depends only on temperature. Color of the downwelling light has no effect on the
output.
Fig. 12.5 Comparisons of downwelling light spectra to spectra of ventral
bioluminescence from the squid Abraliopsis pacificus. A stepped spectrum
represented by the dark line is produced at 8°C and resembles the deep-sea
downwelling spectrum, dashed line; both peak at 472 nm. A stepped spectrum shown
by the light line is produced at 23°C, much closer to the spectrum, dotted line, for
moonlight at 20 m in the tropical ocean.
(^) (After Young & Mencher 1980.)
These countershading mechanisms are not perfect, and many mesopelagic fish
depend upon the imperfection. The eyes of many species look upward (Fig. 12.6). It
would be the reduction of visual neural signals caused by shadows from animals
overhead that initiates feeding responses. Not only do eyes of many deep-sea fish
permanently look up, in many the mouth opens upward for grabbing prey above them.
Probably only those shadows small enough to grab safely are attacked.
Fig. 12.6 The mid-water fish Opisthoproctus. Note the upward-directed eyes with
spherical lenses (that are yellow in life). The flat ventral “sole” of this fish is a light
organ.
(^) (Drawing from Cohen 1964.)